It is known that the spectrum of pure-tone back-up alarms is not optimal for providing sound cues which aid perceptual localization in terms of the time taken to localize the sound source (e.g. the reversing vehicle). Pure-tone here means that the fundamental frequency (or pitch) of the alarm signal has a high level compared with other frequency components, and also (generally) that the spectral profile of the alarm signal has a strong harmonic content, with harmonics being related to the fundamental by integer multiples.
Localization of pure-tone alarm signals where the fundamental frequency is above approximately 2 kHz is less accurate than for pure-tone signals with lower fundamental frequencies because the phase-locking of hair-cells in the human cochlea is reduced above this frequency, and also because interaural sound cues based on interaural phase are ambiguous above that frequency with a half wave-length equal to the interaural distance. Furthermore, pure tone frequencies may not be heard due to absorption from plants and other objects, and may also be confused with other audible alarm cues. A third reason why pure-tone alarm signals are not optimal for safety is because of hearing loss factors which workers may suffer (especially in work environments where such back-up system are used, such as mining operations or other industrial work places).